An existing wireless local area network (WLAN) standard that is based on an orthogonal frequency division multiplexing (OFDM) technology includes gradually evolving versions such as 801.11a, 802.11n, and 802.11ac. In an existing WLAN system based on the 802.11a, 802.11n, and 802.11ac standards, a length of a used OFDM symbol is 4 μs, including a cyclic prefix (CP) or a guard interval (GI) of 0.8 μs.
802.11a, 802.11n, and 802.11ac packets each begin with a legacy preamble according to a time sequence, and the legacy preamble further includes three parts: an L-STF (legacy short training field), an L-LTF (legacy long training field), and an L-SIG (legacy signal field). The L-SIG includes two pieces of indication information, that is, a rate and a length, the rate indicates one of eight types of rates defined in the 802.11a protocol, and each type of rate is corresponding to a different modulation and coding format, and the length indicates an amount of transmitted data in the packet with a unit of an octet. In the 802.11a, 802.11n, and 802.11ac packets, the legacy preamble is closely followed by a data field, an HT-SIG field (high throughput signal field), and a VHT-SIG-A field (very high throughput signal-A field) respectively, where the HT-SIG and the VHT-SIG-A each include two OFDM symbols and have a length of 8 μs. By using different modulation schemes, the 802.11a, 802.11n, and 802.11ac standards may well coexist in a same WLAN.
Currently, the IEEE 802.11 standardization organization has started standardization work of a new generation WLAN standard 802.11ax called an HEW (High Efficiency WLAN, high efficiency wireless local area network). OFDMA and uplink MU-MIMO are two main key technologies of the 802.11ax standard. Therefore, a solution is expected to enable 802.11ax devices to be well recognized by each other in a WLAN.